Contracting skeletal muscle generates sounds by an unknown mechanism. Muscle sounds will be studied to determine the fundamental mechanisms responsible for sound generation and to use these sounds in the investigation of muscle injury, muscle fatigue, and muscular control of prosthetic devices. Sounds recorded at the skin surface over isometrically contracting biceps brachii muscles of humans are low frequency signals with a high dynamic range and signal-to-noise ratio. In addition to in vivo recordings, sounds will be recorded in vitro from frog gastrocnemius and sartorius muscles mounted in an acoustically isolated chamber. Data will be obtained using specially produced microphones with 1:1 impedence match, high sensitivity, and good frequency response. Mechanism. Sounds will be recorded from single muscle fibers as well as whole muscles. If single fibers produce sounds, and if the sounds from whole muscles are equivalent to a superposition of single fiber sounds, then interactions between muscle fibers and/or connective tissue do not contribute significantly to the sound signal. Further anatomic localization will be made by mapping the field distribution of acoustic power which will place constraints on the physical configuration of sound sources. Sounds will be compared from active and passive contraction, concentric, isometric, and eccentric contraction, tetanic contraction, contraction with various temperatures and conduction velocities, and during electrical-mechanical dissociation to determine which physiologic variables affect sound production. Muscle injury. Muscle sounds will be recorded during exercise protocols that are known to produce muscle injury. Muscle damage which may occur as a result of sudden failures within muscle fibers may be audible as discrete acoustic events. Fatigue. Muscle sounds can be used clinically to monitor the dissociation of mechanical from electrical events in fatiguing muscle. By monitoring fatigue in patients with reinnervating muscles, we propose to optimize rehabilitation exercise programs while minimizing muscle damage from overexertion. Prosthetics. Muscle sounds have been shown to be capable of controlling a prosthetic hand. Modifications to improve sensitivity and signal-to-noise ratio will be made to the protoype prosthesis and the prosthesis will be used in clinical trials.